1. Field of the Invention
This invention relates generally to volume rendering for computer-aided diagnosis (CAD) in medical imaging. More specifically, the present invention is directed to systems and methods of determining sampling rates for volume rendering based on regional variations, regional and accumulated opacities, and quality settings.
2. Background Discussion
Direct volume rendering is an important technique to visualize three-dimensional (3-D) datasets from medical and other sources. With the advance of the computation power and medical devices, the sizes of the 3D volumetric datasets are expanding quickly. The Visible Human project from the National Library of Medicine produced massive datasets with the size of 512×512×1728 (864 MB). Typical Computed Tomography (CT) machines from Siemens® Medical Solutions can generate a pixel image dataset at a size of 512×512×6000 (3 GB). The capacity to visualize such datasets with high interactivity and high image quality is helpful to medical professionals in diagnosing disease.
Direct volume rendering is a well studied problem in the visualization community. Raycasting, splatting, shearwarp and 3D texture mapping are some of the most popular volume rendering techniques. Among these technologies, raycasting can generate the highest quality images and the algorithms scale well with the increase of the dataset size. Rendering quality can be improved in the ray-casting technique by increasing the sampling rate in 3 dimensions. However, interactivity is difficult to achieve using the brute-force approach due to the intensive memory access and computational nature of the algorithm.
In order to speed up ray-cast rendering, techniques like space leaping and early ray termination can be used. They are particularly effective if the transfer function used for rendering indicates a high degree, or a high proportion, of empty space (totally transparent) or occluded space by an opaque object (early ray termination). On the contrary, if the transfer function reveals a high degree, or high proportion, of partial translucency but no completely transparent regions, space leaping and early ray termination techniques are not very effective. Therefore, for translucent transfer functions, even with these optimization methods, ray-casting can be very slow. If super-sampling is used for higher resolution rendering, performance will be even slower and therefore, interactivity is no longer possible.
Therefore, it would be an advancement in the state of the art to provide a system and method of efficient interactive volume rendering of large datasets that does not sacrifice image quality.